Gasolines manufactured to contain a higher concentration of aromatics such as benzene, toluene and xylenes (BTX) can adequately meet the octane requirements of the marketplace for a high octane fuel. Aromatics, particularly benzene, are commonly produced in refinery processes such as catalytic reforming which have been a part of the conventional refinery complex for many years. However, their substitution for the environmentally unsuitable lead-based octane enhancers is complicated by environmental problems of benzene. Environmental and health related studies have raised serious questions regarding the human health effects of benzene. The findings suggest that exposure to high levels of benzene should be avoided with the result that benzene concentration in gasoline to enhance octane number is limited and controlled to a relatively low value.
When hydrocarbons boiling in the gasoline boiling range are reformed in the presence of a hydrogenation-dehydrogenation catalyst, a number of reactions take place which include dehydrogenation of naphthenes to form aromatics, dehydrocyclization of paraffins to form aromatics, isomerization reactions and hydrocracking reactions. The composition of the reformer effluent or reformate is shifted toward higher octane value product. Catalytic reforming primarily increases the octane of motor gasoline by aromatics formation but without increasing the yield of gasoline.
Reformates can be prepared by conventional techniques by contacting any suitable material such as a naphtha charge material boiling in the range of C.sub.5 or C.sub.6 up to about 380.degree. F. (193.degree. C.) with hydrogen in contact with any conventional reforming catalyst. Typical reforming operating conditions include temperatures in the range of from about 800.degree. F. (427.degree. C.) to about 1000.degree. F. (538.degree. C.), preferably from about 890.degree. (477.degree. C.) up to about 980.degree. F. (527.degree. C.), liquid hourly space velocity in the range of from about 0.1 to about 10, preferably from about 0.5 to about 5; a pressure in the range of from about atmospheric up to about 700 psig (4900 kPa) and higher, preferably from about 100 (700 kPa) to about 600 psig (4200 Kpa); and a hydrogen-hydrocarbon ratio in the charge in the range from about 0.5 to about 20 and preferably from about 1 to about 10.
The treatment of a reformate with crystalline aluminosilcate zeolites is known in the art and has included both physical treatments such as selective adsorption, as well as chemical treatments such as selective conversion thereof. In U.S. Pat. No. 3,770,614 to Graven a process combination is described for upgrading naphtha boiling range hydrocarbons by a combination of catalytic reforming and selective conversion of paraffinic components to enhance yield of aromatic hydrocarbons by contact with crystalline aluminosilicate catalyst having particular conversion characteristics. In U.S. Pat. No. 3,649,520 to Graven a process is described for the production of lead free gasoline by an integrated process of reforming, aromatics recovery and isomerization including C.sub.6 hydrocarbons upgrading to higher octane product for blending.
U. S. Pat. No. 3,767,568 to Chen, incorporated herein by reference, discloses a process for upgrading reformates and reformer effluents by contacting them with specific zeolite catalysts so as to sorb methyl paraffins at conversion conditions and alkylate a portion of aromatic rings contained in the reformates.
Recently, a process has been developed to overcome some of the foregoing challenges in the reformulation of gasoline. The process is known in the art as the Mobil Benzene Reduction (MBR) process and is closely related to the Mobil Olefins to Gasoline (MOG) process. The MBR and MOG processes are described in U.S. Pat. No. 4,827,069 to Kushnerick, U.S. Pat. No. 4,950,387 and U.S. Pat. No. 4,992,607 to Harandi, and U.S. Pat. No. 4,746,762 to Avidan, all of common assignee. These patents are incorporated herein by reference.
The MBR process is a fluid bed process which uses shape selective, metallosilicate catalyst particles, preferably ZSM-5, to convert benzene to alkylaromatics using olefins from sources such as FCC or coker fuel gas, excess LPG, light FCC naphtha or the like. Benzene is converted, and light olefin is also upgraded to gasoline concurrent with an increase in octane value. Conversion of light FCC naphtha olefins also leads to substantial reduction of gasoline olefin content and vapor pressure. The yield-octane uplift of MBR makes it one of the few gasoline reformulation processes that is actually economically beneficial in petroleum refining.
The MBR process as practiced has relied upon light olefin as alkylating agent for benzene to produce alkylaromatic, principally in the C.sub.7 -C.sub.9 range. The light olefins generally preferred include C.sub.2 -C.sub.7 olefins. However, under MBR alkylation conditions the conversion rate for the alkylation of benzene is lower than would be expected. The reason for this lower conversion rate is the reactivity of light olefins to form oligomers. The oligomerization reactions occurs at a rate competitive with alkylation and so diminishes the alkylation conversion rate.
It is an object of the present invention to provide a process to lower the benzene content of gasoline while enhancing octane value by alkylation of benzene using a method that substantially increases the rate of conversion of benzene to alkyl aromatics.
A further object of the invention is to provide the foregoing improved gasoline having a lower Reid vapor pressure, a lower olefin content and a lower sulfur content.